The video streaming, data transmission, and remote control in underwater call for high speed (Gbps) communication link with a long channel length (~10 meters). We present a compact and low power consumption underwater wireless optical communication (UWOC) system utilizing a 450-nm laser diode (LD) and a Si avalanche photodetector. With the LD operating at a driving current of 80 mA with an optical power of 51.3 mW, we demonstrated a high-speed UWOC link offering a data rate up to 2 Gbps over a 12-meter-long, and 1.5 Gbps over a record 20-meter-long underwater channel. The measured bit-error rate (BER) are 2.8 × 10−5, and 3.0 × 10−3, respectively, which pass well the forward error correction (FEC) criterion.
© 2016 Optical Society of America
There is an increasing demand for high transmission speed, large bandwidth underwater data communication links to meet the rapid growing human activities underwater [1,2]. For example, the oceanography studies, ocean seismic monitoring, sea floor survey, pollution control, and offshore oil exploration will require the utilization of various underwater networks, including distributed sensor nodes, unmanned underwater vehicles (UUVs) and autonomous underwater vehicles (AUVs), as shown in Fig. 1. In order to enable real-time video streaming, data upload and remote control for those applications, an underwater wireless communication technology with high data rate and transmission speed becomes essential [3, 4].
Conventionally, the acoustic communications has been widely used for underwater applications owing to a relatively low attenuation, but the bandwidth of the underwater acoustic channel is limited to tens of kHz, making it less favorable in data-intensive applications [1, 5]. Besides, the slow propagation of sound waves also leads to a substantial delay in the communication link. The radio frequency (RF) communications is another technology benefiting from its high transmission speed of electromagnetic wave and a relatively large bandwidth of few MHz underwater. However, the high attenuation of RF waves in seawater significantly limits the effective transmission distance to a few meters . Recently, the underwater wireless optical communication (UWOC) technology has attracted considerable attention as an alternative solution for short-range marine communication links owing to the large modulation bandwidth, low latency, energy efficiency and small footprint properties [1, 4, 7–15]. Table 1 summarizes the advances of UWOC systems with system performance, including the light source, modulation technique, channel length, and the achieved data rate.
It is apparent that the laser diodes (LD) outperform light-emitting diodes (LED) in data rate when comparing the light sources used in UWOC system. This is further supported by the demonstrated high data rates from LD based free-space visible light communication (VLC) systems [16, 17]. Data rates of 1~2 Gbps have reported in LD based UWOC system by directly modulating the LD using the straightforward on-off-keying (OOK) modulation scheme. UWOC systems with higher data rates were achieved by employing complex modulation schemes, such as orthogonal frequency-division multiplexing (OFDM). However, the channel length for the demonstrated LD based UWOC link is relatively short (in the range of 0.3 ~7 m) and the attenuation effect for LD based UWOC system in ~10 meters has not yet been well studied. As UWOC link over an underwater channel of > 10 meters will usher in flexibility and pervasiveness in field applications (Fig. 1), it is important to develop a high-speed and long-range UWOC system using LDs. In this paper, we report on the study of high-speed LD based UWOC link offering a data rate up to 2 Gbps over a 9-meter /12-meter-long, and 1.5 Gbps over a record 20-meter-long underwater channel. The 450 nm laser diode is operating at a relatively small driving current of 80 mA with an optical output power of 51.3 mW. The bit-error rate (BER) for the UWOC system at 2 Gbps over a 9-m, at 2 Gbps over a 12-m, and at 1.5 Gbps over a 20-m underwater channel is measured to be 1.2 × 10−6, 2.8 × 10−5, and 3.0 × 10−3, respectively, all passing the forward error correction (FEC) limit. Our demonstrated LD based UWOC system using the least complex and the most cost-effective OOK modulation technique extends the UWOC link to 20 meters while maintaining its high data transmission rate.
2. Experimental details
Figure 2 presents the schematic of the experimental setup for laser based underwater optical wireless communications using non-return-to-zero, on-off-keying (NRZ-OOK) modulation scheme. At the transmitter side, a TO-38 packaged single-mode blue-emitting laser diode (Osram PL 450B) with collimation lens is mounted on a thermoelectric cooler (TEC) module (Thorlabs LDM9LP). The laser diode is driven using a Keithley 2400 source meter as the direct current (DC) source. The pattern generator in the J-BERT (Keysight N4903B) is used to generate the pseudorandom binary sequence (PRBS) 210-1 pattern, and the data stream is amplified by the 28-dB driver amplifier (Tektronix PSPL 5868) before connecting with the 15 GHz broadband bias-tee (Tektronix PSPL 5580). At the receiver side, a plano-convex lens (Thorlabs LA1027-A) is used to focus the light into the Si avalanche photodetector (Menlo Systems APD210). A DC blocking filter (Thorlabs T8535) was connected to the APD. The eye diagram is collected using a digital communications analyzer (Keysight 86100C DCA), and the BER is measured using J-BERT (Keysight N4903B). A parametric network analyzer (Keysight E8361C PNA) is used for the small-signal bandwidth measurement. The laser diode characterization was performed at room temperature. The setup involves a diode laser testing system (Keithley 2520) for the voltage vs. current and optical power vs. current measurement as well as a high-resolution spectrometer (Ocean Optics HR 4000) for the spectrum measurement. The water channel in this study was filled with tap water. The transmission spectrum of tap water sample was measured using a UV-Vis spectrophotometer (Thermo Fisher Scientific Evolution 600).
3. Results and discussions
The transmission spectrum of the water sample is shown in Fig. 3. Significant attenuation is identified in the wavelength regime of < 350 nm (UV) as well as > 700 nm (IR). There is a relatively flat transmission window in the blue-green-yellow color regime, and the transmission band of 99% maximum transmission (0.99Tmax) is measured to be 395 nm ~670 nm, which is consistent with reported absorption spectrum . Therefore, UWOC system using laser diodes with emission in violet-blue-green-yellow color regime is expected to benefit from a minimized spectral beam attenuation.
The light output power – current – voltage (LIV) characteristics of the laser diode (LD) used in this study is shown in Fig. 4(a). The LD shows a threshold current of 24 mA, corresponding to a voltage of 4.3 V. At the injection current of 80 mA, the LD has an output optical power of 51.3 mW. The series resistance and slope efficiency for the LD is calculated to be 16.3 Ω and 0.91 W/A, respectively. The electroluminescence (EL) spectra of the LD at different current injections are plotted in Fig. 4(b). A narrow emission peak with a full width at half maximum (FWHM) of < 1 nm and peak wavelength of ~449 nm was measured. There is insignificant EL peak shift with increasing injection current from 40 mA to 120 mA. Thus, the transmission system will be less likely to be affected by the increasing driving current of the LD. As a result, the water channel distance can be extended by increasing the output power of the LD, which will bring insignificant change to the emission peak related channel transmission.
As a figure of merit, the small signal frequency response of the system is measured as shown in Fig. 5(a), which takes into account of both the transmitter, i.e. the laser diode (LD), and the receiver, i.e. the photodetector (PD). A −3 dB bandwidth of ~1 GHz is measured. With increasing current of 60 mA to 100 mA, there is an insignificant change of frequency response. This is different from the behavior of an LD-limiting system, in which the bandwidth is extended as the bias current increased due to up-shift of the relaxation oscillation frequency and gain . Considering the fact that the photodetector has a bandwidth of 1 GHz, the modulation bandwidth of the system is therefore PD-limiting. Nonetheless, the modulation bandwidth of the presented LD based UWOC system is more than ten times higher than those in LED based system . Before the UWOC measurement, a throughput optimization of the operation point was performed by adjusting the input to the LD, including the injection current and the modulation signal amplitude. For the NRZ-OOK modulation of the LD, the changes of BER at different operating current [Fig. 5(b)] and at a different peak-to-peak voltage from the pattern generator [Fig. 5(c)] were measured at a data rate of 1.5 Gbps. Generally, a higher DC current will lead to a higher resonance frequency of the LD, and therefore a better BER performance would be expected. However, it should be noted that an overly biased operation of a laser diode also declines the throughput response. As a result, the BER performance will degrade as observed in Fig. 5 (b) when the driving current goes beyond 80 mA. At low driving voltage, the relative low signal-to-noise ratio may lead to a high BER. However, if the driving voltage is too high, the signal may also be affected by the transient heating. The minimized BER is identified at the driving current of 80 mA and encoding amplitude of 230 mV, where the LD exhibits the optimized performance for high-speed data communication applications.
Aiming at achieving beyond 10-meter LD-based UWOC link, we first studied the BER performance as a function of the received optical power at 9-meter [Fig. 6(a)] and 12-meter [Fig. 6(b)] transmission distance when the data rate of the UWOC system varies from 1 Gbps to 1.5 Gbps and 2 Gbps. The received optical power was attenuated using a series of neutral density filters (Thorlabs, NEK01) at the receiver side. With increasing water channel length, an increasing signal attenuation is expected in the UWOC link, as a result of scattering and absorption. Therefore, the study of BER vs. received optical power is an important approach to evaluating the minimum required received optical power for the LD based UWOC system to achieve a certain data rate at a certain transmission distance. For instance, at a 9-m underwater channel, a received optical power of −19 dBm is required to achieve a BER of 1.0 × 10−3, which is below the FEC limit (3.8 × 10−3), at 1 Gbps. When the data rate increases to 2 Gbps, a minimum received optical power of −16 dBm is required to obtain the similar performance. This is attributed to an increased signal-to-noise ratio (SNR) in the UWOC link, which can be observed from the eye diagrams showing the data transmission at 1 Gbps [Fig. 6 (c)] and 2 Gbps [Fig. 6 (e)]. The BER values are way below 10−8 for data rate up to 1.5 Gbps, which is observed in both 9-meter and 12-meter UWOC link. At 2 Gbps, the system BER is 1.2 × 10−6 for 9-meter UWOC link and 2.8 × 10−5 for 12-meter UWOC link, which is well below the FEC limit, confirming the potential for LD based UWOC system to enable Gbps data transmission link beyond 10 meters.
As the optical power in the UWOC system is one of the critical factors affecting the data rate, we hereby investigate the further extending of the transmission distance, a UWOC channel of 20 meters as an example, by analyzing the attenuation coefficient of the water channel. The received optical power through the water channel can be determined by the Beer–Lambert law ,
where I0 is the transmitted optical power at the transmitter side before entering the water channel; c is the attenuation coefficient and x is the channel distance. The attenuation effect is also observed from a reducing received signal amplitude in the eye diagrams when comparing those measured from a 9-meter UWOC link [Fig. 6(c) and 6(e)] and those from a 12-meter UWOC link [Fig. 6(d) and 6(f)]. Based on the received optical measurement for the 9-meter and 12-meter UWOC link, the attenuation coefficient can then be derived as,
Therefore, the calculated received optical power for a 20-meter UWOC link is 19 µW (−17.2 dBm). According to the measured BER vs. received optical power relations in Fig. 6, the presented LD based UWOC system is expected to achieve a data rate of 1.5 Gbps for a 20-meter underwater channel. The channel transmission characteristics, such as the Mie scattering, will be affected by the concentration and average size of the moving particulates. For a highly scattered medium, such as harbor water, the transmitted optical signal will suffer from an increasing attenuation coefficient.
Figure 7(a) shows the implementation of the LD-based UWOC system for a 20-meter underwater channel. In order to extend the channel length, a broadband dielectric mirror (Thorlabs BB4-E02) is used in the setup to reflect the laser beam. The distance between the laser diode/ photodetector and the mirror is 10 meters, so the total UWOC link is 20 meters. The measured BER at the date rates of 1 Gbps, 1.25 Gbps, and 1.5 Gbps is shown in Fig. 7(b). At a data rate of 1.5 Gbps, a BER of 3.0 × 10−3 is measured, which passes the FEC limit. This is consistent with the presented calculation. Besides, the transmission can be further improved by introducing equalizers at the receiver to reduce the inter-symbol interference. Open eyes are observed for 20-meter UWOC at 1 Gbps and 1.5 Gbps as in insets of Fig. 7(b). To the best of our knowledge, this is the highest data rate achieved for underwater wireless optical communication link exceeding 10 meters.
In summary, we have demonstrated a laser diode based underwater wireless optical communication (UWOC) system with a data rate of 1.5 Gbps over a 20-meter underwater channel based on a simple NRZ-OOK modulation scheme. The optical transmission measurement of the water sample suggests that the GaN-based laser diodes emitting in the violet-blue-green-yellow color regime are better transmitters for UWOC system owing to the minimized attenuation. The presented UWOC system utilizing a single-mode 450-nm LD as the transmitter and optimized biasing condition, showed a −3 dB bandwidth of ~1GHz, with an attenuation coefficient of 0.085 m−1 in tap water. Open eye diagrams and measured FEC compliant BER for a data rate of up to 2 Gbps were successfully achieved for a 9-meter as well as a 12-meter UWOC link. The experimental study of UWOC link with various transmission distances up to 20 meters confirmed that the LD based UWOC system is a practical and promising platform technique for high-speed data communications applications underwater, beyond basic demonstration.
Qatar National Research Fund (QNRF) grant through National Priority Research Program (NPRP) No. 8-648-2-273.
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